1. Field of the Invention
The present invention relates to an output transistor protection circuit for use in a power supply circuit, audio power amplifier, or the like.
2. Description of the Prior Art
In a power supply circuit, audio power amplifier, or the like, an output transistor may be subjected to momentary overcurrent when, for example, the load is in trouble. Therefore, in such circuits, it is essential that the output transistor be protected from destruction by some protection circuit.
FIG. 1 shows an example of a conventional output transistor protection circuit. In this protection circuit, the transistor T10 serves as an output transistor, and its collector is connected to an output terminal 30 and is also connected through resistors R11 and R12 to ground. The node a between these resistors R11 and R12 is connected to the base of the transistor T6.
In this protection circuit, the transistors T1 to T8 constitute a comparator 31. This comparator 31 compares the voltage Va at the node a with a reference voltage Vref, which is applied to the base of the transistor T1. The output current I.sub.1 from the comparator 31 is fed through the transistors T7 and T8 to the base of the output transistor T10 as negative feedback. Accordingly, the comparator 31 serves to keep the voltage Va at the node a equal to the reference voltage Vref. Numeral 32 represents a constant current source.
The transistor T11 serves as a monitor transistor, and its base and emitter are connected to the base and emitter, respectively, of the output transistor T10. The monitor transistor T11 is so formed that, when a current I flows through the output transistor T10, a current 1/n times as high as that current I (i.e. a current I/n) flows through the transistor T11. The collector of the monitor transistor T11 is connected through a resistor R13 to ground and is also connected directly to the base of the npn-type transistor T9.
This protection circuit feeds its output current Io via the output terminal 30 to a load (not shown). As the output current Io increases, the current I increases accordingly, and so does the current I/n, which flows through the transistor T11 and the resistor R13. As a result, the voltage Vb across the resistor R13 rises above the threshold voltage of the transistor T9 and thereby turns on the transistor T9.
In normal condition, the transistors T2 and T3 are balanced, so that the transistor T3 feeds a constant current to the base of the transistor T7. However, once the transistor T9 is turned on, the base current of the transistor T7 decreases accordingly, and so does the current I.sub.1, which flows through the transistor T8. As a result, the base currents of the transistors T10 and T11 decrease, and thus both of the currents I and I/n decrease. In this way, overcurrent is suppressed.
As described above, in the conventional protection circuit, overcurrent is prevented by turning on the transistor T9 and thereby decreasing the drive current (the base current, in this case) to the output transistor T10. However, since this protection circuit uses negative feedback, it attempts to keep its balance and thus to operate linearly even when the transistor T9 is turned on. As a result, this protection circuit exhibits an operation characteristic as shown in FIG. 2. As seen from FIG. 2, the conventional protection circuit operates too slowly to achieve effective protection against momentary overcurrent (such as rush current). In FIG. 2, the output current Io is taken along the horizontal axis, and the output voltage Vo is taken along the vertical axis.
Moreover, in the conventional protection circuit, the output current Io is at its maximum when the output voltage Vo is clamped at a comparatively low level as indicated by A in FIG. 2. In addition, since the voltage between the collector and emitter of the transistor T10 equals Vcc-Vo, this voltage becomes higher as the output voltage Vo at the node A becomes lower. This means that the transistor T10 is in some cases subjected simultaneously to overvoltage and overcurrent, each of which is on its own damaging to any transistor. Thus, the conventional protection circuit exposes the transistor 10 to higher risk of destruction.